Curiously that service is shown on RTT» as reversing at Bristol Parkway. And indeed it is allowed 10 mins from departing BPW» to passing FAW. Of course a reversal at BPW itself isn't possible, and I wouldn't have thought it would have time to reverse at Patchway Jn or Filton West Jn (and what would be the reason?), so it's a bit puzzling.
I read it that it leaves BPW at 20:39 (has to - to get clear of following Swansea train) then sits for 7 minutes pathing allowance on the corner round to Filton Abbey Wood to let the Cardiff to Taunton service that calls at Filton at 20:42 go ahead.